The Mre11 complex, composed of Mre11, Rad50 and Nbs1 subunits (MRN), is essential for the maintenance of genome stability. Nbs1 and Mre11 are linked to the Nijmegen breakage syndrome (NBS) and ataxia-telangiectasia-like disorder (ATLD), respectively, and the affected patients are predisposed to cancer. The Mre11 complex plays a critical role in DNA damage response and DNA double-strand break (DSB) repair, but the underlying mechanisms are not fully understood. In this study, we identified new functions of the Mre11 complex in common fragile site protection, replication fork protection and DSB repair. We propose to further investigate the mechanisms underlying the role of the Mre11 complex in preserving genome integrity and promoting DNA DSB repair in mammalian cells. First, we will determine the role of the Mre11 complex in the protection of common fragile site stability. We will use newly established assays to examine common fragile site protection and explore the mechanisms of MRN to maintain fork stability and repair DSBs generated at common fragile sites. Second, we will study the function of the Mre11 complex to protect stalled replication forks and to promote repair-coupled replication restart at collapsed forks through specific interactions with other fork stabilizing proteins. Third, we will investigate how the Mre11 complex modulates end resection at DSB ends and regulates the utilization of appropriate pathways to repair DSBs. These studies will reveal the molecular mechanisms underlying the critical functions of the Mre11 complex in the maintenance of genome stability and will provide insights into the molecular basis of how MRN deficiency leads to cancer in affected individuals and how maintenance of genome stability contributes to the prevention of tumorigenesis in humans.